Selaginella arenicola and related species have been considered as forming a species complex. This interpretation has been the center of much taxonomic controversy (R. M. Tryon 1955; G. P. Van Eseltine 1918). Tryon recognized one species in the complex, S . arenicola , with three subspecies: subsp. arenicola , subsp. riddellii , and subsp. acanthonata . Other authors (e.g., R. T. Clausen 1946) treated the subspecies as species. I recognize two well-defined species within this complex, S . arenicola and S . acanthonota , which are readily distinguishable by the characteristics given in the key. Some specimens reported by R. M. Tryon (1955) as intermediate between S . arenicola and S . acanthonota appear to be hybrids between S . acanthonota and S . rupestris . In particular, more detailed studies are needed to assess whether populations from Georgia are hybrids or variants of S . acanthonota or of S . rupestris . Future studies are also needed to determine relationships and proper taxonomic rank of Selaginella arenicola subsp. arenicola and subsp. riddellii , which are provisionally recognized here.